Accelerometers require either a frequency or voltage reference. Voltage references hardened against radiation are not available so that frequency referenced accelerometers are preferred for strategic applications. Frequency based accelerometers include silicon micromachined devices and quartz devices. Because of limitations in making small quartz beams, the quartz accelerometers are large and made of several hand-assembled pieces, whose joints cause performance-limiting errors. Silicon micromachining offers smaller size and lower cost in more reliable monolithic accelerometers but suffer less than desired performance, thermal sensitivity and fabrication yield.
These accelerometers typically have a proof mass suspended above a substrate. A tuning fork is connected to the substrate through an anchor at one end and to the proof mass at the other end. The tuning fork has a certain mechanical resonance with no acceleration applied. A force current supplied to the tuning fork causes a sense current whose frequency is a function of that mechanical resonance. When an acceleration occurs the proof mass moves relative to the substrate causing the tuning fork to be stretched or compressed depending on the direction of acceleration along the input axis. Stretching increases the stiffness and thus the frequency of the sensed current. Compression decreases stiffness and the frequency. This change in frequency can be used to measure the acceleration. Often two tuning forks are used arranged so that when one is stretched the other is compressed. Frequency shift between the two improves the representative signal and suppresses common mode errors. One improvement on these accelerometers employs a force multiplier in the form of a lever and a second anchor to multiply the force of the acceleration applied to the tuning fork. In both approaches errors occur with variation in temperature due to two different error sources, Young's modulus and the coefficient of thermal expansion. An increase in temperature causes the elements to expand, and increase the tension and, hence, stiffness and thus the frequency of the tuning fork and sense current. But an increase in temperature causes Young's modulus to decrease in silicon and stiffness decreases with decreases in Young's modulus. A decrease in stiffness results in a drop in frequency of the sense current. The two effects have opposing effects on the accuracy of the tuning fork.